Chapter 22: Object-Based Databases Database System Concepts, 6th Ed. ©Silberschatz, Korth and Sudarshan See www.db-book.com for conditions on re-use.
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Chapter 22: Object-Based Databases
Database System Concepts, 6 th Ed
.
©Silberschatz, Korth and Sudarshan See www.db-book.com
for conditions on re-use
Chapter 22: Object-Based Databases
Complex Data Types and Object Orientation Structured Data Types and Inheritance in SQL Table Inheritance Array and Multiset Types in SQL Object Identity and Reference Types in SQL Implementing O-R Features Persistent Programming Languages Comparison of Object-Oriented and Object-Relational Databases
Database System Concepts - 6 th Edition 22.2
©Silberschatz, Korth and Sudarshan
Object-Relational Data Models
Extend the relational data model by including object orientation and constructs to deal with added data types.
Allow attributes of tuples to have complex types, including non-atomic values such as nested relations.
Preserve relational foundations, in particular the declarative access to data, while extending modeling power.
Upward compatibility with existing relational languages.
Database System Concepts - 6 th Edition 22.3
©Silberschatz, Korth and Sudarshan
Complex Data Types
Motivation: Permit non-atomic domains (atomic indivisible) Example of non-atomic domain: set of integers,or set of tuples Allows more intuitive modeling for applications with complex data Intuitive definition: allow relations whenever we allow atomic (scalar) values — relations within relations Retains mathematical foundation of relational model Violates first normal form.
©Silberschatz, Korth and Sudarshan Database System Concepts - 6 th Edition 22.4
Example of a Nested Relation
Example: library information system Each book has title, a list (array) of authors, Publisher, with subfields
name
and
branch
, and a set of keywords Non-1NF relation
books
Database System Concepts - 6 th Edition 22.5
©Silberschatz, Korth and Sudarshan
4NF Decomposition of Nested Relation
Suppose for simplicity that title uniquely identifies a book In real world ISBN is a unique identifier Decompose
books
into 4NF using the schemas: (
title, author, position
) (
title, keyword
) (
title, pub-name, pub branch
) 4NF design requires users to include joins in their queries.
©Silberschatz, Korth and Sudarshan Database System Concepts - 6 th Edition 22.6
Complex Types and SQL
Extensions introduced in SQL:1999 to support complex types: Collection and large object types Nested relations are an example of collection types Structured types Nested record structures like composite attributes Inheritance Object orientation Including object identifiers and references Not fully implemented in any database system currently But some features are present in each of the major commercial database systems Read the manual of your database system to see what it supports
©Silberschatz, Korth and Sudarshan Database System Concepts - 6 th Edition 22.7
Structured Types and Inheritance in SQL
Structured types
(a.k.a.
user-defined types
) can be declared and used in SQL
create type
Name
as
(first
name
varchar
(20),
lastname
varchar
(20))
final create type
Address
as
(
street
varchar
(20),
city zipcode
varchar varchar
(20), (20))
not final
Note:
final
and
not final
indicate whether subtypes can be created Structured types can be used to create tables with composite attributes
create table
person
(
name Name, address Address, dateOfBirth
date
) Dot notation used to reference components:
name.firstname
©Silberschatz, Korth and Sudarshan Database System Concepts - 6 th Edition 22.8
Structured Types (cont.)
User-defined row types create type
PersonType
as
(
name Name, address Address, dateOfBirth
date
)
not final
Can then create a table whose rows are a user-defined type
create table
customer
of
CustomerType
Alternative using
unnamed row types
.
create table
person_r
(
name address
row(
first
name
varchar
(20),
lastname
varchar
(20)),
row(
street
varchar
(20),
city
varchar
(20),
dateOfBirth zipcode
varchar
(20))
,
date
)
©Silberschatz, Korth and Sudarshan Database System Concepts - 6 th Edition 22.9
Methods
Can add a method declaration with a structured type.
method
ageOnDate
(
onDate
date
)
returns interval year
Method body is given separately.
create instance method
ageOnDate
(
onDate
date
)
returns interval year for
CustomerType
begin return
onDate
-
self
.
dateOfBirth
;
end
We can now find the age of each customer:
select
name.lastname, ageOnDate
(
current_date
)
from
customer
Database System Concepts - 6 th Edition 22.10
©Silberschatz, Korth and Sudarshan
Constructor Functions
Constructor functions
are used to create values of structured types E.g.
create function
Name
(
firstname
varchar
(20),
lastname
varchar
(20))
returns
Name
begin set self
.
firstname = firstname;
set self.
lastname
=
lastname;
end
To create a value of type
Name,
new
Name
(‘John’, ‘Smith’) we use Normally used in insert statements
insert into
Person
values
(
new
Name
(‘John’, ‘Smith),
new
Address
(’20 Main St’, ‘New York’, ‘11001’),
date
‘1960-8-22’);
©Silberschatz, Korth and Sudarshan Database System Concepts - 6 th Edition 22.11
Type Inheritance
Suppose that we have the following type definition for people:
create type
(
name address Person
varchar(20), varchar(20)) Using inheritance to define the student and teacher types
create type
(
under
Person degree
varchar(20),
department
varchar(20))
create type under
(
Student Teacher Person salary
integer,
department
varchar(20)) Subtypes can redefine methods by using overriding method in place of method in the method declaration
©Silberschatz, Korth and Sudarshan Database System Concepts - 6 th Edition 22.12
Multiple Type Inheritance
SQL:1999 and SQL:2003 do not support multiple inheritance If our type system supports multiple inheritance, we can define a type for teaching assistant as follows:
create type under
Teaching Assistant Student, Teacher
To avoid a conflict between the two occurrences of rename them
department
we can
create type under
Teaching Assistant Student
with (
department
as
student_dept
),
Teacher
with (
department
as
teacher_dept
) Each value must have a
most-specific type Database System Concepts - 6 th Edition 22.13
©Silberschatz, Korth and Sudarshan
Table Inheritance
Tables created from subtypes can further be specified as
subtables
E.g.
create table
people
of
Person;
create table
students
of
Student
under
people;
create table
teachers
of
Teacher
under
people;
Tuples added to a subtable are automatically visible to queries on the supertable E.g. query on
people
also sees
students
and
teacher
s.
Similarly updates/deletes on
people
on subtables also result in updates/deletes To override this behaviour, use “
only
people”
in query Conceptually, multiple inheritance is possible with tables e.g.
teaching_assistants
under
students
and
teachers But is not supported in SQL currently
So we cannot create a person (tuple in
people
) who is both a student and a teacher
©Silberschatz, Korth and Sudarshan Database System Concepts - 6 th Edition 22.14
Consistency Requirements for Subtables
Consistency requirements on subtables and supertables.
Each tuple of the supertable (e.g.
people)
can correspond to at most one tuple in each of the subtables (e.g.
students
and
teachers)
Additional constraint in SQL:1999: All tuples corresponding to each other (that is, with the same values for inherited attributes) must be derived from one tuple (inserted into one table). That is, each entity must have a most specific type We cannot have a tuple in
people
corresponding to a tuple each in
students
and
teachers
Database System Concepts - 6 th Edition 22.15
©Silberschatz, Korth and Sudarshan
Array and Multiset Types in SQL
Example of array and multiset declaration :
create type
Publisher
(
name
as varchar
(20),
branch
varchar
(20));
create type
Book
as
(
title
varchar
(20),
author_array
varchar
(20)
array
[10],
pub_date
date
,
publisher Publisher
,
keyword-set
varchar
(20)
multiset
);
create table
books
of
Book;
Database System Concepts - 6 th Edition 22.16
©Silberschatz, Korth and Sudarshan
Creation of Collection Values
Array construction
array
[‘Silberschatz’,`Korth’,`Sudarshan’] Multisets
multiset
[‘computer’, ‘database’, ‘SQL’] To create a tuple of the type defined by the books relation: (‘Compilers’,
array
[`Smith’,`Jones’],
new
Publisher
(`McGraw Hill’,`New York’),
multiset
[`parsing’,`analysis’ ]) To insert the preceding tuple into the relation books
insert into
books
values
(‘Compilers’,
array
[`Smith’,`Jones’],
new
Publisher
multiset
(`McGraw Hill’,`New York’), [`parsing’,`analysis’ ]);
©Silberschatz, Korth and Sudarshan Database System Concepts - 6 th Edition 22.17
Querying Collection-Valued Attributes
To find all books that have the word “database” as a keyword,
select
title
from
books
where ‘
database’
in
(
unnest
(
keyword-set
)) We can access individual elements of an array by using indices E.g.: If we know that a particular book has three authors, we could write:
select
author_array
[1],
author_array
[2],
author_array
[3]
from
books
where
title
= `Database System Concepts’ To get a relation containing pairs of the form “title, author_name” for each book and each author of the book
select
B.title, A.author
from
books
as
B
,
unnest
(
B.author_array
)
as
A
(
author
) To retain ordering information we add a
with ordinality
clause
select
B.title, A.author, A.position
from
books
as
B
,
unnest
(
B.author_array
)
with ordinality as
A
(
author, position
)
©Silberschatz, Korth and Sudarshan Database System Concepts - 6 th Edition 22.18
Unnesting
The transformation of a nested relation into a form with fewer (or no) relation-valued attributes us called
unnesting
.
E.g.
select
title
,
A
as
author
,
publisher.name
as
pub_name
,
publisher.branch
as
pub_branch
,
K.keyword
from
books
as
B
,
unnest
(
B.author_array
)
as
A
(
author
)
,
unnest
(
B.keyword_set
)
as
K
(
keyword
)
Result relation
flat_books
Database System Concepts - 6 th Edition 22.19
©Silberschatz, Korth and Sudarshan
Nesting
Nesting
is the opposite of unnesting, creating a collection-valued attribute Nesting can be done in a manner similar to aggregation, but using the function
colect
() in place of an aggregation operation, to create a multiset To nest the
flat_books
relation on the attribute
keyword
:
select
title
,
author
,
Publisher
(
pub_name, pub_branch
)
as
publisher
,
collect
(
keyword
)
as
keyword_set
from
flat_books
groupby
title, author, publisher
To nest on both authors and keywords:
select
title
,
collect
(
author
)
as
author_set
,
Publisher
(
pub_name, pub_branch
)
as
publisher
,
collect
(
keyword
)
as
keyword_set
from
flat_books
group by
title
,
publisher
©Silberschatz, Korth and Sudarshan Database System Concepts - 6 th Edition 22.20
Nesting (Cont.)
Another approach to creating nested relations is to use subqueries in the
select
clause, starting from the 4NF relation
books4
select
title
,
array
(
select
author
from
authors
as
A
where
A.title
=
B.title
order by
A.position
)
as
author_array
,
Publisher
(
pub-name, pub-branch
)
as
publisher
,
multiset
(
select
keyword
from
keywords
as
K
where
K.title = B.title
)
as
keyword_set
from
books4
as
B
Database System Concepts - 6 th Edition 22.21
©Silberschatz, Korth and Sudarshan
Object-Identity and Reference Types
Define a type
Department
with a field
name
and a field
head
which is a reference to the type
Person,
with table
people
as scope:
create type
Department
(
name
varchar
(20),
head
ref
(
Person
)
scope
people
) We can then create a table
departments
as follows
create table
departments
of
Department
We can omit the declaration
scope
people from the type declaration and instead make an addition to the
create table
statement:
create table
departments
of
Department
(
head
with options scope
people
) Referenced table must have an attribute that stores the identifier, called the
self-referential attribute create table
people
of
Person
ref is
person_id
system generated; ©Silberschatz, Korth and Sudarshan Database System Concepts - 6 th Edition 22.22
Initializing Reference-Typed Values
To create a tuple with a reference value, we can first create the tuple with a null reference and then set the reference separately:
insert into
departments
values
(`CS’, null)
update
departments
set
head
= (
select
p.person_id
from
people
as
p
where
name
= `John’)
where
name
= `CS’
Database System Concepts - 6 th Edition 22.23
©Silberschatz, Korth and Sudarshan
User Generated Identifiers
The type of the object-identifier must be specified as part of the type definition of the referenced table, and The table definition must specify that the reference is user generated
create type
Person
(
name
varchar
(20)
address
varchar
(20))
ref using varchar
(20)
create table
people
of
Person
ref is
person_id
user generated
When creating a tuple, we must provide a unique value for the identifier:
insert into
people
(
person_id, name, address
) (‘01284567’, ‘John’, `23 Coyote Run’)
values
We can then use the identifier value when inserting a tuple into
departments
Avoids need for a separate query to retrieve the identifier:
insert into
departments
values
(`CS’, `02184567’)
©Silberschatz, Korth and Sudarshan Database System Concepts - 6 th Edition 22.24
User Generated Identifiers (Cont.)
Can use an existing primary key value as the identifier:
create type
Person
(
name
varchar
(20)
primary key
,
address
varchar
(20))
ref from
(
name
)
create table
people
of
Person
ref is
person_id
derived
When inserting a tuple for
departments
, we can then use
insert into
departments
values
(`CS’,`John’)
Database System Concepts - 6 th Edition 22.25
©Silberschatz, Korth and Sudarshan
Path Expressions
Find the names and addresses of the heads of all departments:
select
head
–>
name
,
head
–>
address
from
departments
An expression such as “head –> name” is called a
path expression
Path expressions help avoid explicit joins If department head were not a reference, a join of with
people departments
would be required to get at the address Makes expressing the query much easier for the user
Database System Concepts - 6 th Edition 22.26
©Silberschatz, Korth and Sudarshan
Implementing O-R Features
Similar to how E-R features are mapped onto relation schemas Subtable implementation Each table stores primary key and those attributes defined in that table or, Each table stores both locally defined and inherited attributes
Database System Concepts - 6 th Edition 22.27
©Silberschatz, Korth and Sudarshan
Persistent Programming Languages
Languages extended with constructs to handle persistent data Programmer can manipulate persistent data directly no need to fetch it into memory and store it back to disk (unlike embedded SQL) Persistent objects:
Persistence by class
- explicit declaration of persistence
Persistence by creation
- special syntax to create persistent objects
Persistence by marking
- make objects persistent after creation
Persistence by reachability
- object is persistent if it is declared explicitly to be so or is reachable from a persistent object
©Silberschatz, Korth and Sudarshan Database System Concepts - 6 th Edition 22.28
Object Identity and Pointers
Degrees of permanence of object identity
Intraprocedure
: only during execution of a single procedure
Intraprogram
: only during execution of a single program or query
Interprogram
: across program executions, but not if data-storage format on disk changes
Persistent
: interprogram, plus persistent across data reorganizations Persistent versions of C++ and Java have been implemented C++ ODMG C++ ObjectStore Java Java Database Objects (JDO)
©Silberschatz, Korth and Sudarshan Database System Concepts - 6 th Edition 22.29
Persistent C++ Systems
Extensions of C++ language to support persistent storage of objects Several proposals, ODMG standard proposed, but not much action of late
persistent pointers
: e.g. d_Ref
creation of persistent objects Class extents
: e.g. new (db) T() : access to all persistent objects of a particular class
Relationships:
Represented by pointers stored in related objects Issue: consistency of pointers Solution: extension to type system to automatically maintain back-references
Iterator interface Transactions Updates:
mark_modified() function to tell system that a persistent object that was fetched into memory has been updated
Query language ©Silberschatz, Korth and Sudarshan Database System Concepts - 6 th Edition 22.30
Persistent Java Systems
Standard for adding persistence to Java :
Java Database Objects (JDO)
Persistence by reachability Byte code enhancement Classes separately declared as persistent Byte code modifier program modifies class byte code to support persistence – – E.g. Fetch object on demand Mark modified objects to be written back to database Database mapping Allows objects to be stored in a relational database Class extents Single reference type no difference between in-memory pointer and persistent pointer Implementation technique based on
hollow objects pointer swizzling
) (a.k.a.
©Silberschatz, Korth and Sudarshan Database System Concepts - 6 th Edition 22.31
Object-Relational Mapping
Object-Relational Mapping (ORM)
relational databases systems built on top of traditional Implementor provides a mapping from objects to relations Objects are purely transient, no permanent object identity Objects can be retried from database System uses mapping to fetch relevant data from relations and construct objects Updated objects are stored back in database by generating corresponding update/insert/delete statements The
Hibernate
ORM system is widely used described in Section 9.4.2
Provides API to start/end transactions, fetch objects, etc Provides query language operating direcly on object model queries translated to SQL Limitations: overheads, especially for bulk updates
©Silberschatz, Korth and Sudarshan Database System Concepts - 6 th Edition 22.32
Comparison of O-O and O-R Databases
Relational systems
simple data types, powerful query languages, high protection.
Persistent-programming-language-based OODBs
complex data types, integration with programming language, high performance.
Object-relational systems
complex data types, powerful query languages, high protection.
Object-relational mapping systems
complex data types integrated with programming language, but built as a layer on top of a relational database system Note: Many real systems blur these boundaries E.g. persistent programming language built as a wrapper on a relational database offers first two benefits, but may have poor performance.
©Silberschatz, Korth and Sudarshan Database System Concepts - 6 th Edition 22.33
End of Chapter 22
Database System Concepts, 6 th Ed
.
©Silberschatz, Korth and Sudarshan See www.db-book.com
for conditions on re-use
Figure 22.05
Database System Concepts - 6 th Edition 22.35
©Silberschatz, Korth and Sudarshan
Figure 22.07
Database System Concepts - 6 th Edition 22.36
©Silberschatz, Korth and Sudarshan